It is generally known that a wheel bearing apparatus rotationally supports a wheel of vehicle with respect to a suspension apparatus incorporates a rotational speed detecting apparatus to detect a rotation speed of a wheel of vehicle to control the anti-lock braking system (ABS). Heretofore, such a wheel bearing apparatus has a sealing apparatus arranged between inner and outer members that are relative rotated, via rolling elements. A magnetic encoder with magnetic poles alternately arranged along its periphery and is integrated with the sealing apparatus. A rotational speed sensor is arranged opposite to the magnetic encoder to detect variation of the magnetic poles of the magnetic encoder according to rotation of a wheel of a vehicle.
In general, the rotational speed sensor is mounted on a knuckle of a suspension apparatus after the wheel bearing apparatus has been mounted on the knuckle. However, in a recent wheel bearing apparatus, it has proposed that the rotational speed sensor is incorporated into the wheel bearing apparatus in order to eliminate complicated work of adjusting an air gap between the rotational speed sensor and the magnetic encoder to reduce the size of the wheel bearing apparatus.
FIG. 4 is one example of the wheel bearing apparatus. This wheel bearing apparatus comprises an inner member 51, an outer member 52, and double row balls 53, 53 accommodated between the inner and outer members 51, 52. The inner member 51 comprises a wheel hub 54 and an inner ring 55 press-fit onto the wheel hub 54 via a predetermined interference. The inner ring 55 is press-fit onto a cylindrical part 54a of the wheel hub 54 and axially secured thereto by a caulked part 56 formed by plastically deforming an end of the cylindrical part 54a. 
A pulser ring 57 is press-fit onto an outer circumference of the inner ring 55. The pulser ring 57 comprises a supporting annulus 58 and a magnetic encoder 59 adhered by vulcanized adhesion to a side surface of the supporting annulus 58.
A cap 60 is fit into an opening of an annular space formed between the outer member 52 and the inner ring 55 to prevent leakage of grease filled in the bearing and entering of rain water or dusts from the outside.
The cap 60 is securely press-fit into an inner-side end of the outer member 52 to close an opening of the outer member 52. The cap 60 is press-formed of austenitic stainless steel sheet as having a cup-shape and comprises a cylindrical fitting part 60a to be press-fit into an inner circumference of an end of the outer member 52 and a bottom part 60c extending radially inward from the fitting part 60a via a radially reduced part 60b to cover the end of the inner member 51.
An elastic member 61 formed of synthetic rubber such as NBR (acrylonitrile-butadiene rubber) is integrally adhered by vulcanized adhesion to an outer circumference of the radially reduced part 60b of the cap 60. The elastic member 61 is adhered to the cap 60 so that it does not project to the inner-side from the side surface of the bottom part 60c of the cap 60 so as not to interfere the rotational speed sensor 62 and the elastic member 61 has an annular projection 61a projected from an outer circumference of the fitting part 60a. The annular projection 61a can be elastically deformed against the inner circumference of the end of the outer member 52 to improve the sealability of fitting part 60a when the cap 60 is fit into the outer member 52.
A nut 64 is securely pressed into a circular recess 63 formed at the center of the bottom part 60c of the cap 60. The rotational speed sensor 62 can be secured on the cap 60 by fastening s securing bolt to a female thread 64a via a mounting flange (not shown). This enables the rotational speed sensor 62 to be smoothly secured without causing an eccentric torque on the cap 60 when fastening the securing bolt.
The rotational speed sensor 62 is arranged closely to or contacted with a side surface of the bottom part 60c of the cap 60 at a position corresponding to the magnetic encoder 59. The wheel speed can be detected by detecting the variation of magnetic flux of the magnetic encoder 59 with the rotational speed sensor 62 via the bottom part 60c of the cap 60. This makes it possible to obtain a desirable air gap and thus to improve the assembling workability of the rotational speed sensor 62 without complicated air gap adjustment as well as to provide a wheel bearing apparatus with a rotational speed detecting apparatus which can secure the sealability without adversely affecting the sensing performance since the magnetic encoder 59 is sealed by the cap 60 (see e.g., JP 2011-196425 A).
In such a wheel bearing apparatus with a rotational speed detecting apparatus of the prior art, since the elastic member 61 is integrally adhered by vulcanized adhesion to an outer circumference of the radially reduced part 60b of the cap 60 and the elastic member 61 has an annular projection 61a projected from an outer circumference of the fitting part 60a, it is superior that the annular projection 61a can be elastically deformed against the inner circumference of the end of the outer member 52 to improve the sealability of fitting part 60a when the cap 60 is fit into the outer member 52.
However, it is believed that, in the wheel bearing apparatus of the prior art, the cap 60 would interfere with the magnetic encoder 59 since the radially reduced part 60b of the cap 60 on which the elastic member 61 is adhered is positioned at a substantially same axial position as that of the supporting annulus 58 on which the magnetic encoder 59 is adhered. Accordingly, it is necessary to increase the outer diameter of the cap 60 or reduce the outer diameter of the supporting annulus 58 in order to avoid the interference between the magnetic encoder 59 and the cap 60, but this limits the freedom of design.
In addition, the cap 60 is press-formed of non-magnetic austenitic stainless steel sheet such as SUS 304 as having a cup-shape so as to have the corrosion resistance and prevent adverse effect on the sensing performance of the rotational speed sensor 62. However, the austenitic stainless steel sheet of this kind has property that its metallic structure tends to be martensitic transformed especially in a largely bent region when it is press-drawn from a steel sheet to the cup-shape and therefore transforms from a non-magnetic member to a magnetic body. Thus, the bent region would be magnetized by magnetism from the outside and it is believed that the sensing performance of the rotational speed sensor 62 would be adversely affected. More particularly, although the steel sheet is not necessarily magnetized with martensitic transformation alone, it is sometimes magnetized due to the transformation of the cap 60 to a magnetic body when the cap 60 is approached to surrounding magnetized components such as dies or tools.
Although it is possible to neglect influences to the sensing performance caused by transformation of the cap 60 to magnetic body when the magnetic force of the magnetic encoder 59 has sufficient allowance relative to the detection limit of the rotational speed sensor 62, the influence to the sensing performance cannot be neglected when the magnetic encoder 59 does not have sufficient allowance relative to the detection limit of the rotational speed sensor 62, i.e. when the magnetic force of the magnetic encoder 59 itself is small or when the air gap between the magnetic encoder 59 and the rotational speed sensor 62 is large.